This invention relates to fluorescent lamp ballasts incorporating an integrated circuit. More particularly, the invention relates to such ballasts including circuitry in addition to the integrated circuit for implementing the functions of end-of-lamp life shutdown, automatic resetting of the ballast when a lamp is replaced, and limiting the number of attempts to start the lamp.
Ballasts, or power-supply, circuits for fluorescent lamps can benefit from incorporating various circuit functions in integrated circuit (IC) form. IC""s can include a driver for a halfbridge switching arrangement that provides AC power for the lamp. Proprietary IC""s typically also include the following, generally-stated functions: (1) end-of-lamp life shutdown; (2) automatic resetting of the ballast when a lamp is replaced, and (3) limiting the number of attempts to start the lamp.
Proprietary IC""s, however, are often not available to a ballast manufacturer. On the other hand, ballast manufacturers can obtain widely used, low cost IC""s incorporating various functions including a half-bridge switching arrangement, but lacking the foregoing three functions. It would be desirable if additional circuitry could be provided to enable the foregoing three functions in conjunction with such low cost IC""s. It would further be desirable if such additional circuitry could be implemented economically.
In a preferred form, the invention provides a fluorescent lamp ballast, comprising a load circuit for at least one lamp that includes an inductance and capacitance for setting a resonant frequency of the circuit. A half-bridge switching arrangement supplies AC current to the load. An integrated circuit comprises a driver for the half-bridge arrangement including control means to create a frequency sweep from a pre-heat frequency, through a substantially lower, resonant frequency, to a still lower operating frequency. A pre-heat pin in the IC triggers the control means to re-start a frequency sweep in response to a first signal exceeding a first threshold level. A shut-down pin in the IC, associated with an internal shut-down latch, shuts down the driver in response to a second signal exceeding a second threshold level. A power-supply pin in the IC provides power to the integrated circuit.
When the ballast is powered-up, the integrated circuit starts a frequency sweep at the pre-heat frequency, substantially above the resonant frequency of the output network, where the voltage across the lamp is below the ignition voltage. The integrated circuit holds the frequency fixed for about 1 second, allowing the lamp filaments time enough to heat prior to ignition. The integrated circuit then drops the frequency relatively rapidly down to the operating frequency, passing through the resonant frequency. In normal operation, the lamp ignites in response to the resonant build-up of voltage. However, if the lamp fails to ignite, the half-bridge switches experience potentially destructive current spikes, caused by operation with no resistive load below resonance. This stressful situation is immediately corrected by pre-heat trigger circuitry that detects the current spikes through switches of the half-bridge switching arrangement and, in response, supplies the pre-heat pin with a first signal exceeding the first threshold level. This triggers the integrated circuit into a new frequency sweepor start-up sequenceommencing with a pre-heat mode, where the frequency is once again above resonance for a dwell time of about 1 second, followed by a frequency drop. This cycle of lamp ignition attempts could continue indefinitely, if not for the end-of-lamp life circuitry. End-of-lamp life circuitry provides to the shut-down pin a second signal exceeding the second threshold level if lamp current fails to reach a substantial portion of its normal level within a predetermined period of time. A DC current-supply path is provided from a DC current supply, through at least one filament of each lamp in the load circuit, to the power-supply pin of the integrated circuit.
The end-of-lamp life circuitry cooperates with the pre-heat trigger circuitry by limiting the number of frequency sweeps and hence lamp ignition attempts-to no more than occur during the predetermined period of time set by the end-of-lamp life circuitry. Limiting the lamp ignition attempts is desirable from the user""s point of view. Each ignition attempt can be accompanied by a flash of light from a defective lamp. If ignition attempts were not limited, the persistent flashes of light could be annoying to the user.
The DC path cooperates with the end-of-life circuitry and the internal shut-down latch to reset the latch when the DC path is broken due to absence of at least one filament in the path. The latch resets when a lamp is removed for replacement with a new lamp. The reset of the latch when a lamp is removed is an important operational feature because, otherwise, the primary power must be removed momentarily to reset the latch, thereby enabling a new lamp to start. Removal of primary power, even momentarily, is inconvenient to the user.
The foregoing ballast provides circuitry in addition to widely used IC""s for providing the functions of: (1) end-of-lamp life shutdown; (2) automatic resetting of the ballast when a lamp is replaced; and (3) a limitation on with the number of attempts to start the lamp. In preferred embodiments, such functions can be implemented especially economically due to cooperation between circuit functions.